1. Field of the Invention
The present invention relates to a coaxial starting motor which has an electromagnetic switch mounted at its rear end.
2. Discussion of Background
Referring to FIG. 2, there is shown an axial cross sectional view of a conventional coaxial starting motor, which has been disclosed in e.g. Japanese Unexamined Patent Publication No. 92573/1989. In FIG. 2, reference numeral 1 designates a d.c. motor which is constructed as follows: Field poles 2 which are made of e.g. a permanent magnet are attached to a yoke 3. Reference numeral 4 designates an armature wherein an armature core 5 with armature windings 6 wound thereon, and commutators 8 are fixed on a hollow rotary shaft 7. Reference numeral 9 designates a front bracket, which is coupled together with the yoke 3, a rear bracket 10 and a casing 41 by means of through bolts 11. Reference numeral 12 designates a brush holder which is attached to the yoke 3, and which presses brushes 13 against the commutators 8. The rotary shaft 7 is supported by a bearing 14 held in a bearing supporting portion 3a formed integrally with the yoke 3, and a bearing 15 held in the rear bracket 10.
Reference numeral 17 designates a planetary reduction gear which is constructed as follows: Reference numeral 18 designates a sun gear which is constructed by a pinion formed on a front end portion of the rotary shaft 7. Reference numeral 19 designates a plurality of planetary gears which mate with the sun gear 18, and which are supported by supporting pins 20 through bearings 21. The supporting pins 20 are fixed to a clutch outer 25 which also works as a carrier. Reference numeral 22 designates an internal gear which is formed on an inner periphery of a front end portion of the yoke 3, and which causes the planetary gears 19 mating therewith to revolve.
Reference numeral 24 designates an overrunning clutch which is constructed by the clutch outer 25, a clutch inner 26 and a bearing 28, the clutch inner 26 transmitting one-direction rotation to the clutch outer 25 through rollers 27. The clutch inner 26 is supported by the front bracket 9 through a bearing 29. Reference numeral 30 designates an output shaft which is coupled with helical spline grooves 26a of the clutch inner 26 at its helical spline portion 30a, and which has a rear end side supported in the rotary shaft 7 through a bearing 31. Reference numeral 33 designates a return spring which works to retract and return the output shaft 30. Reference numeral 34 designates a pinion which is carried on a front end portion of the output shaft 30 by spline connection, and which is received by a stopper 35. Reference numeral 36 designates a buffer spring which urges the pinion 34.
Reference numeral 40 designates an electromagnetic switch which is mounted to a rear end of the d.c. motor 1 to be coaxial therewith, and which is constructed as follows: A casing 41 which forms a magnetic path has a fixed core 42 and a magnetic path core 43 fixed thereto. Reference numeral 44 designates an exciting coil which is wound on a bobbin 45. Reference numeral 46 designates an end cover which is made of a non-magnetic material. Reference numeral 47 designates a movable core which forms a plunger, and which is movably guided, in an axial direction, by a guide sleeve 48 fixed to the magnetic path core 43 and the fixed core 42. Reference numeral 49 designates a movable rod which is made of a tube of a non-magnetical material, and which has the movable core 47 fixed at a rear end. Reference numeral 50 designates a movable contact which is supported by the movable rod 49 through an insulating supporter 51, and which is urged by a pressing spring 52. Reference numeral 53 designates a pushing rod which has a rear end portion supported to be movable in the rod 49 in an axial direction, and which has a front end pushed against a bore bottom in a rear end of the output shaft 30 through a steel ball 54. Reference numeral 55 designates a receiving member which is fixed in the rear end of the movable rod 49. Reference numeral 56 designates a return spring for the movable rod 49. Reference numerals 57 and 58 designate compression springs. Reference numeral 59 designates an insulating holder which is fixed to the rear bracket 10, and which is molded from synthetic resin. Reference numeral 60 designates a pair of fixed contacts which are embedded in the insulating holder 59. Reference numeral 61 designates a terminal bolt for connecting to a power source, which is made integrally with one of the fixed contacts 60, and which projects outwardly.
In the conventional starting motor stated earlier, when a vehicle starting switch is closed, the exciting coil 44 is energized. The movable core 47 moves forwardly by magnetic attraction to push the movable rod 49, causing the output shaft 30 to travel forwardly through the pushing rod 53. As a result, the pinion 34 gets into mesh with an engine ring gear. In addition, the movable contact 50 gets in touch with the paired fixed contact 60 to energize the electrical circuit of the d.c. motor 1, causing the armature 4 to rotate. The rotation of the rotary shaft 7 given by the armature 4 is transmitted to the output shaft 30 through the planetary reduction gear 17 and the overrunning clutch 24 to start an engine by the pinion 34. When the engine has started and the starting switch is opened, the output shaft 30 is retracted by the return spring 33, and the movable rod 49 is also retracted by the return spring 56, causing the movable contact 50 to separate from the fixed contacts to break the energization of the d.c. motor 1.
The conventional coaxial starting motor as stated earlier involves a problem in that the arrangement wherein the movable contact 50 and the fixed contacts 60 of the electromagnetic switch 40 are positioned in a displacement chamber of the movable core 47 allows contact powder to adhere on a sliding surface of the periphery of the movable core 47 to obstruct the sliding movement of the movable core. The conventional coaxial starting motor also has a problem in that the presence of a gap between the movable core 47 and the guide sleeve 48 permits the movable rod 49 to incline, causing the movable rod 49 to get in touch with the internal wall of the hollow space of the rotary shaft 7 in some instances. There is a possibility that when the movable rod 49 rotates depending on the rotary shaft 7, the movable contact 50 could rotate or separate from the fixed contacts 60.